A seat-belt apparatus for restraining a vehicle occupant has a webbing take-up device. Among the webbing take-up devices, there are so-called tension reducer mechanisms for mitigating or canceling a feeling of excessive pressure which a vehicle occupant suffers from when the vehicle occupant puts on the webbing, pretensioner mechanisms for eliminating slight looseness referred to as “slack” by causing the take-up shaft to take up a predetermined amount of a webbing belt at the time of a sudden deceleration of a vehicle, to increase the restraining force applied to a vehicle occupant's body by the webbing belt, and to hold a vehicle occupant's body with increased certainty. Further, there is known a so-called motor retractor which carries out the aforementioned functions by using a motor (see Patent Documents 1 and 2, as an example).
Such a motor retractor is quite useful because the motor retractor can implement the same functions as the aforementioned tension reducer or pretensioner can do, and also help the device take up and retract webbing at the time when the webbing is ordinarily fitted onto a vehicle occupant.
Particularly in recent years, in such a motor retractor as described above, a structure is considered in which a distance from a vehicle on which a vehicle occupant is riding to another vehicle or obstacles in front is detected by a forward-observation device such as a distance sensor, and when the distance between the vehicle and another vehicle or obstacles in front becomes less than a predetermined value, a motor is operated, and due to rotation force of the motor, a take-up shaft is rotated in a take-up direction.
Meanwhile, in the case of such a motor retractor as described above, in order to prevent transmission of rotation from the take-up shaft to the motor, a clutch is interposed between the output shaft of the motor and the take-up shaft. Accordingly, the clutch transmits only the rotation from the output shaft of the motor.
However, in such a conventional motor retractor, there has been a need to dispose a clutch opposite to an end portion of the take-up shaft. Therefore, the retractor has been structured such that a clutch device is disposed at the outer side of a pair of leg plates constituting a frame, a motor is disposed at the outer side of the leg plate at which the clutch is disposed (in other words, the same as the clutch), and the motor is disposed at the upper side or the lower side of the frame with respect to the pair of the leg plates. However, in such cases, since a comparative large and heavy part like the motor is positioned at the outer side of the frame and at the upper side or at the lower side, there have been drawbacks in that the entire body of the motor retractor becomes large, and the weight balance of the motor retractor itself becomes bad.
Also there is a desire that it be easy in such a motor retractor to change and set a gear ratio (deceleration ratio) of the driving force to be transmitted from the motor to the clutch, a rotational speed to be transmitted to the clutch when the motor is driven, and a rotational speed of the take-up shaft (set variations in the webbing take-up characteristics).
A certain webbing take-up device comprises a spool around which a webbing is taken up, a pretensioner mechanism for taking up the webbing around the spool in a vehicle emergency, and a detection mechanism for detecting a vehicle acceleration and a webbing pull-out acceleration from the spool (see Patent Document 3 as an example).
In the webbing take-up device, a pretensioner mechanism is disposed at one end portion of the spool, while a detection mechanism is disposed at the other end of the spool to exhibit excellent arrangement balance (spatial balance) of the mechanism between one end and the other end of the spool.    Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 2001-130376    Patent document 2: JP-A No. 2001-347923    Patent document 3: JP-A No. 8-133015